Heart failure remains a leading cause of morbidity and mortality in the United States. While the complexity and heterogeneity of the disease has been increasingly elucidated over the last decade, limited progress in its medical therapy has been made. This is particularly the case for heart failure with preserved ejection fraction (HFpEF), which accounts for 30-50% of all cases. We hypothesize here that distinct changes in myofiber architecture play a fundamental role in the pathogenesis of heart failure. We further hypothesize that diffusion tensor MRI (DTI) of the human heart in vivo will allow these changes to be characterized, thereby elucidating the microstructural basis of myocardial remodeling and heart failure. Our group has previously developed a diffusion-encoded stimulated echo pulse sequence to perform cardiac DTI in vivo. We will enhance the speed, coverage, and accuracy of the sequence to allow those with cardiac disease to be comfortably and reliably imaged. This will be accomplished through simultaneous multislice excitation, free-breathing approaches, and tailored spatiotemporal registration techniques. To investigate changes in myocardial architecture, we will use advanced analysis methods tailored to cardiac DTI that our group has developed over the last few years. To portray microstructural alterations, the supertoroidal model of the diffusion tensor will be used since it is highly sensitive to small changes in myocardial microstructure. For architectural analysis, the following tractography- based indices will be used: (i) propagation and torsion angles, (ii) the fiber architecture matrix, and (iii) shet tractography. We hypothesize that these tools will reveal subtle changes in the structure and organization of myofibers in the heart, providing novel insight into disease progression and identifying new targets for therapy. Aim 1 of the proposal will involve ongoing development and optimization of the in vivo DTI pulse sequence, using the aforementioned approaches and methods. In Aim 2, we will use the developed techniques to explore the microstructural changes seen in patients with cardiac remodeling after myocardial infarction. Preliminary data reveal that significant abnormalities in fiber and sheet architecture occur in both the border and remote zones in patients shortly after infarction. We will follow these changes longitudinally to better understand their relationship with the development of left ventricular dilation and heart failure with reduced ejection fraction (HFrEF). Aim 3 will focus on detecting microstructural changes associated with an increased risk of HFpEF. The age- and load-dependence of DTI indices in the left ventricle will be characterized in normal volunteers. Thereafter, diabetic patients with and without (i) hypertension, (ii) left ventricular hypertrophy, and (iii) a recent admission for HFpEF will be studied. The proposal extends the boundaries of cardiac imaging by providing DTI-based phenotypic biomarkers that will characterize the remodeling process at the microstructural level and predict the development of heart failure. These advancements will play a central role in the diagnosis, prevention and evaluation of new therapies for heart failure, which will be of major public health significance.